badgering purchaser rant!

Have you ever been approached to sell your car, that doesn't have a for sale sign on it, and you tell the person you would be willing to listen to a reasonable offer and the person says to you, HOW MUCH YOU WANT!
My standard response is I don't know, make me an offer that's reasonable and we can go from there. These people just cannot come out with a price, so they badger you to no ends about how much you want for it! I have a tendency to not be very friendly when the badgering keeps going on. My end all discussion response after all this badgering is put a price on the item that is so far overpriced, I should be locked up!
Is this a common negotiating tactic? just ignorance? just kicking tires? or I'm old and cranky!
My view is, if your going to ask, you should have an idea of what you are willing to offer. I can politely say no thank-you instead of telling you to,,,,,,, after your badgering!